1. Field of the Invention
This invention relates to a digital-to-analog converter (hereinafter referred to as D/A) that converts multi-bit digital data into an analog signal by dividing the digital data into more significant bit data and less significant bit data, converting them into respective analog signals and then carrying out analog addition of the converted analog signals.
2. Description of Related Art
Conventionally, a multi-bit D/A converter is known which is formed of a resistor string and CMOS transistors. This resistor string D/A converter has a high output impedance, and thus, its output current must be received by a high input impedance circuit. For this reason, an adder formed by an operational amplifier is ordinarily provided at an output stage of the D/A converter.
Although the above resistor string D/A converter performs D/A conversion with high accuracy, it has a disadvantage that the increase in the number of bits to be converted results in increase in the number of resistors used. This causes the increase in the size of an integrated circuit that constitutes the D/A converter because a large space is required for the resistors.
To overcome this disadvantage, there has been proposed a circuit as disclosed by Japanese Laid-Open Patent Publication (Kokai) No. 8-307273. FIG. 3 shows an example of the circuit arrangement of a D/A converter disclosed in this publication. In this circuit arrangement, a conventional resistor string D/A conversion section 11 converts fourteen more significant bit data of 16-bit digital data into a corresponding analog voltage signal (hereinafter referred to as more significant analog voltage signal), and a current mirror circuit 12 converts two less significant bit data of the 16-bit digital data into corresponding analog current signals (hereinafter referred to as less significant analog current signals). An adder 13 and a current switching circuit 14 convert the analog current signals corresponding to the less significant bit data into a corresponding analog voltage signal (hereinafter referred to as less significant analog voltage signal), and at the same time, add the less significant analog voltage signal to the more significant analog voltage signal. The resulting signal is then outputted from an output of an operational amplifier of the adder 13 as an output signal from the D/A converter.
With this arrangement in which all of the 16-bit digital data are converted into the corresponding analog voltage signals, the above D/A converter can reduce its size compared with the D/A converter comprised of the resistor string and the CMOS transistors and make it easier to increase the number of bits for conversion.
The above conventional D/A converter, however, has the necessity of presetting a resistance value r12 of a resistor R12 and current values i2, i3 of currents I2, I3 flowing through transistors Q2, Q3 according to a set value of voltage (reference voltage)of a reference power supply to the D/A converter, which is inputted from the outside. More specifically, the adder 13 must add the more significant analog voltage signal and the less significant analog voltage signal on correct analog voltage levels corresponding to respective bit positions. This necessitates presetting the resistance value r12 and the current values i2, i3 so that voltage values corresponding to the two less significant bit data are xc2xd and xc2xc of a voltage value of least significant bit data of the more significant bit data that is converted by the D/A conversion section 11. It is therefore difficult for a user to change the reference voltage as desired once the D/A converter as a whole is designed. Thus, the conventional D/A converter lacks flexibility.
It is an object of the present invention to provide a digital-to-analog-converter which allows a use to set the reference voltage as desired and therefore has improved flexibility.
To attain the above object, the present invention provides a digital-to-analog converter comprising a more significant data converting device that converts more significant bit data of digital data into a more significant analog signal corresponding to the more significant bit data by using a reference voltage inputted from outside, a less significant data converting device that converts less significant bit data of the digital data into a less significant analog signal corresponding to the less significant bit data, a control device that controls the less significant data converting device in such a manner as to output the less significant analog signal according to the reference voltage, and an adding device that adds the more significant analog signal and the less significant analog signal, thereby outputting an analog signal corresponding to the digital data.
With the above arrangement of the present invention, the control device can automatically output the less significant analog signal according to the reference voltage which is a reference voltage of the more significant analog signal. As a result, the user can set the reference voltage as desired, and thus, the digital-to-analog converter according to the present invention has improved flexibility.
Preferably, the more significant data converting device comprises a voltage output device that outputs a more significant analog voltage signal having a voltage corresponding to the more significant bit data by using the reference voltage.
Also preferably, the less significant data converting device comprises a current output device that outputs a less significant analog current signal having a current corresponding to the less significant bit data.
More preferably, the control device comprises a regulating device that detects a predetermined voltage within the reference voltage and regulates a current value of the less significant analog current signal according to the detected voltage.
Further preferably, the adding device comprises a current-to-voltage converting device that converts the less significant analog current signal into a less significant analog voltage signal and adds the less significant analog voltage signal to the more significant analog voltage signal.
With these preferable arrangements of the present invention, the current of the less significant analog current signal can be automatically adjusted according to the reference voltage which is a reference voltage of the more significant analog voltatage signal, and as a result, the user can set the reference voltage as desired, whereby the flexibility of the digital-to-analog converter can be improved. Further, the more significant data converting device, which is a voltage-mode digital-to-analog converter generally having a high conversion accuracy, can convert the more significant bit data into the more signiciant analog voltage signal, while the less significant data converting device, which is a current-mode digital-toanalog converter generally having a small circuit area, can convert the less significant bit data into the less significant current signal, and therefore highly accurate digital-to-analog conversion can be achieved with a reduced circuit circuit area.
Preferably, the voltage output device comprises a more significant reference voltage terminal supplied with a more significant reference voltage of the reference voltage, a less significant reference voltage terminal supplied with a less significant voltage of the reference voltage, and a resistor string having one end thereof connected to the more significant reference voltage terminal and another end thereof connected to the less significant reference voltage terminal, and having a plurality of outputs, for dividing the reference voltage in divided voltages corresponding in number to a number of bits of the more significant bit data and outputting the divided voltages from the plurality of outputs.
More preferably, the regulating device comprises to a power supply at a high potential side, a MOS transistor having one end thereof connected to the power supply, a resistor having one end thereof connected to the MOS transistor and another end thereof connected to the other end of the resistor string, a differential circuit having a first input connected to a junction between the MOS transistor and the resistor, a second input connected to a predetermined one of the plurality of outputs of the resistor string, and an output connected to a gate of the MOS transistor, and a current sinking device that sinks a current flowing into the other end of the resistor string from the MOS transistor through the resistor, from the one end of the resistor string.
With these prefeerable arrangements of the present invention, the current sinking device can sink the current flowing into the lower end of the resistor string from the MOS transistor through the resistor. As a result, the more significant and less significant reference voltages can be reliably supplied even from the resistors which have low current driving ability, without an extra current flowing through the resistor string, which can further improve the flexibility of the digital-to-analog converter.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.